This invention is an improvement in positive displacement pumps having intermeshing gears. Various kinds of positive displacement pumps are known in the fluid machinery art including vane pumps wherein a pump rotor rotates in a pumping cavity and carries on its periphery pumping vanes, slipper pumps, having loosely mounted slippers carried by a pump rotor in a pumping cavity, and internal-external gear pumps wherein one pumping element is an external gear and the companion meshing gear element is an internal gear. My invention comprises a positive displacement pump which, unlike the previously described positive displacement pump, comprises two external gears that mesh in their respective pump cavities. Examples of such external gear pumps are shown, for example, in U.S. Pat. No. 2,746,394 (Dolza et al); U.S. Pat. No. 2,996,999 (Trautman); and U.S. Pat. No. 2,871,794 (Mosbacher).
In a gear pump of the kind shown in the above-idenified prior art patents, the cavities between the external gear teeth and the surrounding pump housing act as fluid pumping chambers as they carry fluid from an inlet port to an outlet port upon rotation of the gears. As in the case of most gearing systems, the helical gear teeth of my improved pump have a contact ratio for the gears that is greater than unity. Thus there is an overlap in the gear tooth engagement pattern as one pair of meshing teeth begins to engage prior to disengagement of the preceding pair of meshing gear teeth. This creates in the space between the meshing gear teeth a volume of fluid which at one instant decreases in volume and at another instant increases in volume. The instant at which the volume changes from a compression mode to an expanding mode corresponds to the instant that the line of action for the meshing gears intersects the line connecting the centers of rotation of the gears.
In order to avoid a hydraulic lock that would reduce pumping efficiency and create pump noise as well as undesirable hydraulic pressure forces in the pump, it is common practice to use an end plate an each axial side of the meshing pump gears and to provide a recess in the end plates to permit discharge of the fluid trapped in the gear tooth space thus providing communication between that space and the adjacent port. This provides a pressure relief that prevents a buildup of pressure in the trapped fluid in the tooth space of the meshing gear teeth.
The improvements of my invention make it possible to eliminate the need for using ported end plates in a gear pump assembly of this kind. This involves a strategic port design so that each tooth space volume between two meshing teeth in a helical gear pump having a form contact ratio greater than unity registers with the cutoff edge of the outlet cavity at one axial end of the tooth at the same instant the opposite end of the same tooth space volume registers with the inlet edge of the inlet port. Thus there is no opportunity for a pressure pulsation to occur in the fluid trapped between two meshing gear teeth. The use of ported plates with pressure recesses at the ends of the pump gears is not required. The assembly thus is simplified, its cost reduced and the space required for a given pump capacity is reduced.